1. Field of the Invention
This invention relates generally to an ophthalmic lens, and more specifically to multifocal ophthalmic lenses that combine both refraction and diffraction to provide an ocular image.
2. Description of the Related Art
Ophthalmic lenses, such as intraocular lenses (IOLs), phakic IOLs, and corneal implants, are used to enhance ocular vision. For instance, IOLs are now routinely used to replace the natural lens of an eye that is removed during cataract surgery. More recently, diffractive IOLs have been advantageously used to reduce lens thickness and correct for presbyopia. For instance, diffractive bifocal lenses divide incident light into two diffractive orders to provide both near and distance vision. The use of diffractive optics in ophthalmic lenses is described by Cohen in U.S. Pat. Nos. 4,881,804; 4,881,805; 4,995,714; 4,995,715; 5,017,000; 5,054,905; 5,056,908; 5,117,306; 5,120,120; 5,121,979; 5,121,980; and 5,144,483, which are all herein incorporated by reference. Freeman also describes the use of diffractive optics in ophthalmic lenses in U.S. Pat. Nos. 4,637,697; 4,641,934; 4,642,112; 4,655,565; and 5,748,282, which are also herein incorporated by reference.
In such lenses, the optic area is generally divided into a plurality of annular zones or echelettes that are offset parallel to the optical axis by predetermined step heights to provide a specific phase relationship between the zones. The term “zone plate,” or “phase plate,” as used herein and as is generally recognized in the art, is defined to be a pattern of concentrically arranged annular zones which is characterized, at least in part, by the step height between zones, the circumferential spacing between zones, and the surface profile of each zone. Zone plates are usually configured to maintain a predefined phase relationship of light passing through the zones. In addition to Cohen and Freeman, Futhey also describes various ophthalmic diffractive lenses, for example, in U.S. Pat. Nos. 4,936,666; 5,129,718; and 5,229,797, herein incorporated by reference.
In one approach, a phase plate or zone plate comprises a plurality of zones in which the optical height of the steps (i.e., the physical height times the difference between the refractive index of the material and the refractive index of the surrounding media) between the individual zones is one-half that of light at a design wavelength in the visible range. In such designs, approximately 80% of the light at the design wavelength is evenly split between zeroth and first diffraction orders, where the zeroth diffraction order is generally considered to be light that is un-diffracted or unaffected by the zone plate. This zone plate configuration is used to produce a bifocal lens in which (1) the zeroth diffraction order produces a first focus or focal point for distant vision and (2) the first diffraction order produces a second focus or focal point corresponding to near or intermediate vision. In addition, chromatic dispersion produced by the first diffraction order, which is usually opposite in sign to refractive chromatic dispersion, may be used to reduce the overall chromatic aberrations in the near vision focus, since the refractive and diffractive chromatic dispersions components tend to cancel one another. However, the distant vision focus does not benefit from this diffractive chromatic dispersion, since it comprises only light that is un-diffracted by the zone plate. Thus, the distance vision is purely refractive and receives no reduction in any chromatic aberrations induced by refractive chromatic dispersions.
A characteristic of ophthalmic lenses incorporating diffractive zones or phase plates is that the amount of light in the near and distant foci is substantially constant for all pupil sizes. It is desirable in certain instances to increase the amount of light in the distant focus as the pupil size increases, for instance under intermediate or low light conditions. One way to increase the amount of light dedicated to distance vision is to restrict the zone plate to the central portion of the lens and to make the outer region of the lens refractive only, as disclosed in Cohen '804. Another approach is disclosed by Lee et al. in U.S. Pat. No. 5,699,142, herein incorporated by reference. Lee et al. teaches a diffractive lens comprising an apodization zone in which the step height between zones in the transition region is progressively reduced. The steps between zones are centered on a base curve BC so as to avoid sharp discontinuities in the resulting wavefront that can produce unwanted diffractive effects. In either of these designs, the outer refractive portion of the lens does not benefit from the use of diffractive power to reduce chromatic aberrations, potentially resulting in increased chromatic aberrations as the pupil size increases under lower lighting conditions.
One problem associated with multifocal/bifocal IOLs is the problem of halos. This problem manifests itself when light from the unused focal image creates an out-of-focus image that is superimposed on the used focal image. For example, if light from a distant point source or slightly extended source is imaged onto the retina of the eye by the distant focus produced by a bifocal IOL, the near focus produced by the IOL will simultaneously superimpose a defocused image on top of the image formed by the IOL's distant focus. This defocused image may manifest itself in the form of a ring of light surrounding the in-focus image produced by the IOL's distant focus.
Devices and method are needed to improve the performance of diffractive lenses in ophthalmic applications.